Fuel cells are receiving increasing attention as a viable alternative energy system. In general, fuel cells convert chemical energy into electrical energy in an environmentally clean and efficient manner, typically via oxidation of hydrogen or an organic fuel in the anodic half-cell coupled to an oxygen reduction reaction (ORR) in the cathodic half-cell. Fuel cells are contemplated as power sources for everything from small electronics to cars and homes. In order to meet different energy requirements, there are a number of different types of fuel cells in existence today, each with different chemistries, requirements, and uses.
Biofuel cells are fuel cells that rely on or mimic natural biological processes to produce power. Examples of biofuel cells include enzymatic fuel cells (EFCs), which use enzymes as the electrocatalysts and microbial fuel cells (MFCs), which use microorganisms for conversion of chemical energy to electricity. In general, the difference between EFCs and MFCs is that EFCs use only the enzymes that occur in nature while MFCs use the entire cell.
FIG. 1 shows a general diagram for an exemplary enzymatic biofuel cell using glucose and oxygen. In general, on the anode side, a fuel such as glucose is oxidized via, for example, a glucose oxidizing enzyme, thereby releasing electrons from the fuel. The electrons are then forced to travel around the electrolyte barrier via a wire, thereby generating an electric current. On the cathode side, oxygen is converted to water via, for example, an oxygen reducing enzyme.
Unlike traditional fuel cells which typically cannot use organic fuels due to carbon monoxide poisoning of their precious metal components, enzyme-based fuel cells are able to use organic compound-based fuels such as sugars and alcohols, which are attractive due to their abundance and relative inexpensiveness. There are a number of enzymes which are useful as biocatalysts in biofuels. Examples include PQQ-dependent dehydogenases, which are known to be naturally occurring oxidizing enzymes, and thus useful as biocatalysts in fuel cell anodes, and multi-copper oxidases (MCOs) which are well known biocatalysts for oxygen reduction and are thus useful in fuel cell cathodes.
The design of enzymatic fuel cells today is based on several decades of research focused on ensuring that each liberated electron is efficiently and rapidly transferred to a solid electrode, either via electron-carrying mediators, or via direct electron transfer from enzyme to electrode. However, today's enzyme fuel cells remain limited in power output and lifetime, and thus are ill-suited to power practical devices or applications.
The gist of the present challenge is to design fuel cells that allow rapid transport of electrons from bound enzymes to the electrode surfaces, and thereby circumvent the rate-limited performance of current designs. More specifically, the cathodes in these fuel cells must be designed to ensure an ever-present three-phase-interface between electrolyte, air and enzyme, a condition which is essential for oxygen-reducing enzymes to turn over at their maximum rate. One difficulty in EFCs is that the enzymes must be immobilized near their respective anodes and cathodes, as, if they are not immobilized, the enzymes will diffuse into the fuel. Furthermore, the system must include a mechanism for fast electron transfer of the liberated electrons to and from the electrodes. There are three main paths for enzyme immobilization onto nanocomposite electrodes: i) physical adsorption; ii) using linkers for covalent attachment of the enzyme; iii) encapsulation of the enzyme in polymeric matrix. All this methods have one main disadvantage, the enzyme is randomly oriented on the electrode surface, which means that any particular enzyme may or may not be capable of performing at its optimum level as either a catalyst or participant in electron transfer. Furthermore, because the orientation is random, the current output is neither consistent nor reproducible from one electrode to another, which is a significant problem for mass production.